A technique for manufacturing a thin film transistor (TFT) by using a thin semiconductor film formed over a substrate having an insulating surface has attracted attention. The technique is widely used for thin film transistors or electronic devices such as electro-optical devices. A silicon-based semiconductor material is known as a material for a thin semiconductor film which can be used in a thin film transistor. Other than the silicon-based semiconductor material, an oxide semiconductor has attracted attention.
Better electric characteristics of a transistor including an oxide semiconductor are required for application to semiconductor devices with higher performance. A technique has been reported in which in a transistor including an oxide semiconductor, for the purpose of uniformity and high-speed operation, hydrogen or deuterium contained in a source and drain electrodes is diffused into an oxide semiconductor layer, whereby the resistance of regions in contact with the source and drain electrodes in the oxide semiconductor layer is reduced (e.g., Patent Document 1).